The present invention relates to an improved method of forming bodies comprising thermoset resins and to the product of the method. This invention is especially suitable for bonding or joining together bodies, such as bodies of extruded elongated strips comprising thermally stable, covalently crosslinked resins. The present invention especially relates to the joining of crosslinked rubber extrudates such as vulcanized elastomeric weatherstrips and trim strips.
Thermoset resins are characterized by a covalently crosslinked, thermally stable network and are in contrast to thermoplastic polymers which soften and flow under heat and pressure. Thermosetting resins usually crosslink under heat although some, such as urethanes and epoxy resins may require little or no heat. Thermosetting can occur simultaneously with polymerization or, as in the case of vulcanized rubbers, after polymerization. It will be appreciated by those skilled in the art that the present invention is broadly applicable to the class of thermoset resins, i.e., thermally stable, covalently crosslinked resin materials. However, to facilitate understanding of the invention, it is described in the context of preferred embodiments relating to extruded vulcanized rubber strips used in the automobile industry, such as weatherstrips, trim strips and glass run channel.
Weatherstrips, trim strips, glass run channel and the like are generally made by extruding uncured, i.e., unvulcanized elastomeric materials to form an extrudate which is then vulcanized and cut into suitable lengths. It is often desirable to join such lengths or strips to provide an end to end joint, a T-joint or a corner. It is also often desirable to further form a length or strip after vulcanizing to, for example, provide a bend therein. It is also often desirable to further form a length or strip after vulcanizing to, for example, provide a bend therein. Disadvantages arise, however, when joining pieces of vulcanized materials together or when bending already vulcanized materials.
One method of joining one end of a weatherstrip to another end or length thereof is to employ both extrusion and molding processes. Generally speaking, this method involves extruding unvulcanized rubber into a desired profile and vulcanizing the rubber by heating it to effect crosslinking. After the extrudate is vulcanized it is cut to the desired length to prepare it for a molding step to join it to another length of molding. For example, for a corner joint, the ends of a pair of extrudates are loaded into a corner mold and unvulcanized rubber is introduced into the mold cavity to form a corner section. The unvulcanized corner material is then heated to vulcanize it and bond it to the already vulcanized extrudate ends. The advantages of using extrusions with molded corners are well known in the field of automotive sealing and weatherstripping and trim. The use of this construction has shown steady increase in automotive sealing since its introduction.
However, the above method has the disadvantage of producing a product with unsightly joint lines at the cut ends of the elongated extrudate. Also, the joint obtained by the method is not as strong as the extrudate itself. The joint lines are due to several causes. First, when the ends of the cured extrusions are loaded into the corner mold, the mold must clamp the ends of the extrusions to withstand the cavity pressure of introduced corner material exhibited on the extrusion, therefore preventing the extrusion from being pushed out of the mold. This clamping action on the extrusion causes the extrusion to displace linearly within the extrusion clamping region. Upon mold opening at the end of the corner molding process, the displaced extrusion relaxes back to its original form due to the elastic memory of the cured rubber. This relaxation takes place unimpeded except at the joint interface between the extrusion and molded corner where the extrusion size relaxation causes stress and a dimensional step to the molded material. This dimensional mismatch causes appearance as well as functional, i.e. sealing, problems.
Another cause of unsightly joint lines is due to a difference in gloss. A gloss mismatch exists between the extrudate and the molded corner. This is due to the dissimilar processes of extrusion and molding. Because the surfaces are joined together at a distinct line, the gloss mismatch is accentuated and is therefore unattractive from an appearance standpoint.
A third cause of unsightly join lines is due to a difference in color between the extrudate molded portions. Color mismatch is unsightly and even when the same material is used for both the extrudate and molded portions of the article, variation of the material state of crosslinking can cause an objectionable color shift.
The present invention solves the above joint line problem by employing a transition zone between vulcanized and unvulcanized rubber. Thus color and/or gloss mismatches are blended over a zone or area, which blending results in a less noticeable joint. Further, the method of this invention avoids bonding unvulcanized rubber to a cut surface of vulcanized rubber. This method joins pieces of previously crosslinked resin at interfaces of uncrosslinked resin thus joining the pieces together with a strong bond by allowing uncrosslinked polymer chains to bridge the bonded area before being crosslinked. The present method also facilitates bending or other forming of an extrudate comprising vulcanized resin. Further understanding of the advantages of the present invention will be had from the following disclosure and claims taken in conjunction with the accompanying drawings.